In optical waveguides, birefringence or non-rotationally symmetrical refractive index profiles can result in different group delay times for the two orthogonal polarizations of a fundamental mode. The resultant polarization mode dispersion PMD can lead to noticeable signal distortions, especially in transmission systems with high data rates (typically from 10 Gbit/s). Small propagation time differences between the signals transmitted in both polarizations cause instances of pulse widening, and propagation time differences of the order of magnitude of the bit duration or more have an effect as intersymbol interference. In order still to be able to transmit good-quality signals with a high channel data rate in fibers with high polarization mode dispersion PMD, the propagation time differences must be compensated for again at the end of the transmission path. “Impact of Polarization Mode Dispersion on 10 Gbit/s Terrestrial Systems Over Non-Dispersion-Shifted Fiber”, B. Clesca et al., Electronic Letters, 31st Aug. 1995, Vol. 31, No. 18, pages 1594–1595, illustrates the influence of polarization mode dispersion PMD on signal quality by measurement of the bit error rate BER in the case of propagation time differences between signals with a transmission rate of 10 Gbit/s.
Temperature changes in the surroundings of the optical waveguide or mechanical oscillations or inhomogeneities influence the birefringence or the symmetry of the refractive index profile and also the polarization transformations in the waveguide. As a consequence, there may be a change in the propagation time differences, and/or in the polarizations in which the two signals arrive at a receiver. A compensation method for compensating for the propagation time differences must therefore continuously measure the change and adapt an actuator in a regulating loop to the present state of polarization.
Various methods or systems for measuring or compensating for the polarization mode dispersion PMD are already known.
E 198 18 699 A1 discloses an arrangement for reducing polarization-mode-dispersion-dictated signal distortions by using a filter method. The light is passed to a photodiode for optoelectronic conversion. The electrical signal is split and fed to different electrical filters. These may be, e.g., bandpass filters with center frequencies at ½, ¼ and ⅛ of the clock frequency (that is to say 5 GHz, 2.5 GHz and 1.25 GHz in the case of 10 Gbit/s). The output voltages or output powers of the bandpass filters are detected. The quality of the data signal can be assessed from the magnitude of the spectral components thus detected. If a propagation time difference of 100 ps occurs in the case of a 10 Gbit/s NRZ (non return to zero) signal between the two principal states of polarization PSP, which corresponds to a shift by approximately 1 bit, then the output signal of the 5 GHz filter is at a minimum. In the case of a propagation time difference of 0 ps, the output signal is at a maximum. Thus, in the course of a regulation in a PMD compensator, an attempt is made via corresponding settings at the PMD compensator to maximize the signal. Since the output signal of the filter rises again in the case of propagation time differences which are greater than a bit duration, unambiguous regulation is no longer possible in this region. Therefore, the abovementioned filters with lower center frequencies are additionally required. These reach their minimum only in the case of correspondingly larger propagation time differences. Consequently, in a PMD compensator, the polarization mode dispersion PMD is firstly subjected to course compensation with the aid of the low-frequency filters and then, if the output signal of the highest-frequency filter becomes unambiguous, the greater sensitivity thereof would be utilized in order to compensate for the polarization mode dispersion PMD to the greatest possible extent and thus to readjust it as early as possible in the case of alterations. What is disadvantageous is that first identifiable distortions of the optical signal must occur before the occurrence of polarization mode dispersion PMD is detected. Moreover, distortions are primarily detected, and these may also have arisen due to effects other than PMD.
A known method for measuring the polarization mode dispersion PMD is based on an arrival time detection. In this method, in the case of an optical NRZ signal, the latter is passed through a polarization scrambler at the start of the transmission path. What is thereby achieved is that the polarization passes through all conceivable states at the start of the path within a short time interval. The signal passes through the transmission fiber and a subsequent PMD compensator. If the combination including the transmission fiber and the PMD compensator has a first-order polarization mode dispersion PMD1, the arrival time of the signal will vary. This variation is proportional to the maximum group delay time difference that occurs and thus to the first-order polarization mode dispersion PMD1. Since the clock recovery of the receiver follows these changes in the arrival time, the signal at the input of the voltage-controlled oscillator (VCO) can be fed to an integrator and the output signal thereof can be utilized in order to determine the first-order polarization mode dispersion PMD1. The frequencies at which the polarization scrambler is driven must not be too high, in order that the clock recovery of the receiver can still follow the changes in the arrival time. The fluctuations in the arrival time, the frequency of which lies in the range of the frequencies used for the polarization scrambler, are evaluated in a targeted manner for the measurement of the polarization mode dispersion PMD.
In contrast to the present invention, this method requires a polarization scrambler at the input of the transmission path. In addition, the method no longer functions if the propagation time differences to be measured approach or even exceed the bit duration, since then the clock recovery no longer functions.
EP 0 798 883 A2 discloses an optical receiver with an equalizer circuit for disturbances caused by polarization mode dispersion PMD. The optical receiver of the incoming signal has a splitting device for separating the TE and TM modes of the incoming signal with a polarization controller, which splits the signal fed to it into two electrical signal components corresponding to the TE and TM modes. The two signal components have a propagation time difference which is caused, e.g., by polarization mode dispersion PMD and corresponds to an impairment of the signal quality. Via multistage decision units and a regulating device, the equalizer circuit supplies a quality measurement of the two signal components, e.g., by determining their bit error rate or with the aid of a minimization method for the high-frequency components contained in the electrical signal components. After the selection of the best signal component, the equalizer circuit outputs a data signal with a minimal bit error rate. Delay devices will compensate for the time difference between the two signal components continuously or in a stepwise manner through control signals of the regulating device. Indications about the measurement of the time delay or the control signals for compensating for the time difference are not given here. In the optical part of the optical receiver, only the polarization planes of the incoming optical signal are influenced a regulating signal proceeding from the regulating device, depending on the quality measurement carried out. A measurement of the time delay over a bit duration likewise cannot be carried out.
“Polarization Mode Dispersion Compensation by Phase Diversity Detection”, B. W. Hakki, IEEE Photonics Technology Letters, Vol. 9, No. 1, January 1997, describes a PMD compensator in which an optical signal having polarization mode dispersion is split into the two principal states of polarization PSP by maximizing the measured phase difference between two pseudo-random data signals with a data rate of 10 Gbit/s. After the determination of the phase difference via a 5 GHz Gilbert mixer, a delay line is adjusted for minimizing the phase difference. A measurement or a compensation of the phase difference, caused by polarization mode dispersion, between the data signals corresponding to the principal states of polarization PSP is also limited to the bit duration in this case.
Various PMD compensators are known just for compensating for the polarization mode dispersion PMD.
WO 00/41344 discloses a PMD compensator which automatically finds two principal states of polarization PSP and directs them, after digital signal processing, onto two orthogonal linear directions of polarization of a beam splitter.
WO 00/45531 discloses another PMD compensator, which compensates the phase of each bit of the two data signals, which are time-offset through polarization mode dispersion, without digital signal processing but via a clock recovery and a phase modulator.
WO 00/03505 furthermore discloses a PMD compensator which has a birefringent substrate and a waveguide, formed on the surface, with electrically conductive electrodes, and in which many different polarization transformations can be set via control voltages at the electrodes during operation in such a way that the polarization mode dispersion PMD of first and higher orders can be compensated for. FIG. 5 of this document describes an adaptive PMD compensator in which a regulation one or, depending on small or large values of the polarization mode dispersion, a number of passband filterings of the signal proceeding from the PMD compensator effects a resetting of the control voltages of the PMD compensator, the resetting being controlled by a regulator.
The object of the present invention, then, is to specify a system and a method for measuring and compensating for the distortions due to polarization mode dispersion of first order and moreover higher orders during the transmission of an optical signal which enable large propagation time differences, such as above a bit duration, to be determined.